Propose an explanation, on an atomic/molecular level, for how the heats of solut

Question

Propose an explanation, on an atomic/molecular level, for how the heats of solution tor the group 2 nitrates changed going down that group. Remember that attractive forces between atoms, ions, or molecules are made (among the products) and broken (among the reactants) during a physical or chemical change. It's the making and breaking of these forces that causes heat to be released or absorbed during that change. When an attraction forms, heat is released; conversely a heat input required to break an attraction. The stronger the attraction is, the more heat is required to break it and the more heat is released when it's formed. The heat that we measure for a physical or chemical change is the difference between the required to break the reactants' attractive forces and the heat released when the products' forces are made. Apply these ideas to the dissolving of a salt in water. Specifically, what attractive forces are broken and made? How do their relative strengths determine the heat of solution and how is that related to your results? The enthalpy change values obtained using a calorimeter of the type employed in this experiment are often slightly lower than the accepted values. Suggest a specific reason why.

Explanation / Answer

4. For group II nitrates, as we go down the group the ions become larger so their attraction to the negative nitrate ions becomes weaker. This therefore reduces the lattice enthalpy for the nitrates down the group due to increasing interionic distance. As we go down the group, the postive ion size gets smaller, this therefore interacts less strongly with the negative nitrate ions and hence polarize less effectively the nitrate ions for decomposition. Therefore, the thermal stability of nitrates increases as we go down the group, that is higher amount of energy is needed to break the electrostatic attractive forces between the postive and negative ions as we go down the group. The heat of solution decreases as we go down the group due to less hydration of ions.

5. The enthalpy change value calculated by calorimeter is lower than accepted value, as some of the heat is absorbed by the calorimeter which is not considered and is taken as a constant in case os calculation.

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